The present invention relates to a logic integrated circuit device formed on a compound semiconductor substrate, and more particularly to such a device which is compatible with a silicon emitter-coupled logic integrated circuit device.
As well known in the art, an integrated circuit device formed on a substrate of compound semiconductor, typically of gallium arsenide, (hereafter a "GaAs IC") attains very high speed operation, and therefore it is desired that a silicon emitter-coupled logic integrated circuit device (Si-ECL IC) which is usually employed in a high speed system is replaced by a GaAs IC.
For the GaAs IC to be compatible with a Si-ECL IC, the following three conditions should be satisfied:
(1) Compatibility in power supply voltage and logic levels. More specifically, the power supply voltage of the GaAs IC should be -5.2V, and its logic levels should be -0.7V (high level) and -1.9V (low level).
(2) Compatibility in logic operation. In particular, true and complementary signals should be produced without a time delay.
(3) Compatibility in a load drive capability. A load of 50.OMEGA. should be driven directly.
The inventor developed a prototype GaAs IC which satisfied the above three conditions, but at the sacrifice of power consumption and a semiconductor chip area. More specifically, some of transistors in the prototype GaAs IC were connected in series between power supply terminals. The power supply voltage applied to the IC was -5.2V, so that each of the series-connected transistors was supplied with a small voltage across its source and drain. Therefore, the threshold voltage thereof had to be designed to be of a shallow value. All the transistors in the prototype GaAs IC were fabricated through the same steps, and thus took the same threshold voltage. As a result, the threshold voltage of an output transistor became shallow inevitably. On the other hand, the output transistor should drive the 50.OMEGA. load and produce an output signal having the ECL level. Therefore, the output transistor is required to have a considerably large current ability. In order for the output transistor to have a large current ability with a shallow threshold voltage, its gate width was made remarkably large. As a result, the output transistor occupied a lot of area on the semiconductor chip.
A large gate width increases the input stray capacitance of the output transistor. For this reason, a plurality of buffer amplifiers were required to drive the output transistor. As a result, much power was consumed by the buffer amplifiers. The amplifiers also increased the chip area.